Drainage basins

Drainage basin: The area drained by a river and its tributaries. A drainage basin is considered to be an open system because water can be added or lost from a drainage basin. Source: The beginning of a river. A river may have multiple sources. The source of a river is normally found in upland mountainous areas. Mouth: The end of a river. A river may end in a lake, but more normally in the sea. Tributary: A small river that flows into a larger river. Confluence: Where two rivers meet. Watershed: The border between two drainage basins. Estuary: The tidal section of a river near the mouth. Channel: The physical confines of the river, encompassing two banks and a bed. Bank: The sides of a river channel. A river channel has two banks. Bed: The bottom of a river channel.

A drainage basin is an open system that has inputs, outputs, stores and transfers (flows).

Inputs: When water is added to a drainage basin.

Precipitation: Any moisture that falls from the atmosphere. The main types of precipitation are rain, snow, sleet, hail, fog and dew.

Interbasin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.

Outputs: When water leaves a drainage system.

Evaporation: The process of water turning from a liquid into a vapour. Evaporation only takes place from a body of water e.g. a lake, puddle or the sea.

Transpiration: The evaporation of water from vegetation.

Evapotranspiration: The combined action of evaporation and transpiration

Interbasin transfer: Water that either naturally (due to the alignment of the rock) or with human involvement (pumps and pipes) moves from one drainage basin to another.

River discharge via channel flow: Water entering the sea and leaving a drainage basin. A very small amount of water also enters the sea via throughflow and groundwater flow (baseflow).

Stores: When water is stationary and not moving in a drainage basin.

Interception: When water is caught and held by vegetation or man-made structures like buildings.

Surface store: When water is held in the surface of the earth. This may be a puddle, a lake or a garden pond.

Soil moisture store: When water is held in unsaturated soil.

Groundwater store: When water is held in saturated ground.

Transfers (flows): When water is moving within a drainage basin.

Stem flow: When intercepted water runs down the trunks and stems of vegetation.

Canopy drip: When intercepted water drips off the leaves of vegetation (drip tip leaves in rainforests are actually designed to allow this to happen).

Throughfall: Precipitation that falls directly through vegetation.

Infiltration: Water that moves from the surface of the earth into the soil below.

Throughflow: Water that travels through unsaturated ground.

Pipeflow: Water that travels through holes left by root systems and animals burrows.

Percolation: Water that travels from unsaturated into saturated ground.

Groundwater flow (baseflow): Water that travels through saturated ground.

Capillary action (or rise): Water that may move upwards towards the surface.

Channel flow: Water that travels in a river.

Surface run-off (overland flow): When water travels across the surface of the earth e.g. down a hill.

Saturated: Ground where the pores are full and can contain no more water. Unsaturated: Ground where there is still space between the pores. Water table: The border between saturated and unsaturated ground. The water table may go up or down. Permeable: Surfaces that allow water to pass through them. Impermeable: Surfaces that do not allow water to pass through them. Pores: Gaps between soil and gravel that water can fill. Aquifer: Rock that can hold water. Aquiclude: Rock that can not hold water. Porous: Rock with pore spaces and cracks in it. Non-porous: Rock with no pore spaces or cracks in it. Condenses: When water vapor turn into water droplets. Water can only condense around condensation nuclei.

Feedback Loops: A system is always trying to maintain a balance (equilibrium). Feedback loops explain when a system (drainage basin) loses and attempts to regain its balance. Positive feedback causes an instability by changing inputs (increase in precipitation). Negative feedback attempts to regain the equilibrium by reducing inputs or possibly increasing outputs. Humans can alter feedback loops by changing the amount of water released from dams, changing the amount of water used or even increasing run-off through deforestation.

System Diagram: Because two dimensional and three dimensional diagrams of drainage basins can be complicated to understand, drainage basin systems are often shown in a systems diagram. Below is an example of a systems diagram, it basically shows all the processes taking place using a series of boxes and arrows. The boxes are usually coloured so that you can clearly see if they are process are an input, output, transfer or store.